1. Field of the Invention
The invention relates to a composite multilayer material, as well as to a method of modifying composite multilayer material systems, especially for improving the surface hardness of such systems.
2. Description of Related Art
The use of composite multilayer materials consisting of steel with an overlay of plain bearing materials (e.g. Al- or Cu-based materials with soft phases such as, for example, tin, lead and/or bismuth) for tribological applications has been known for many years. Conventional methods such as cladding by rolling, cladding by casting, sintering or electroplating have proved the best methods of producing such composite systems, but new processes such as sputtering, vapour deposition or plasma coating also constitute prior art.
All these methods of producing such composite multilayer materials are distinguished by the fact that, after coating of the steel backing material with a sliding material, either metal working or surface machining processes are carried out. This is usual in the case of cladding by rolling, cladding by casting, sintering and plasma coating. Often, forming processes are also carried out prior to application of the final overlay, as is the case with electroplating, sputtering and vapour deposition. In none of these manufacturing processes for producing such composite multilayer materials are modifications carried out on the steel backing material of the final, finished component with the aim of optimising the former's tribological properties, such that corresponding disadvantages arise in certain instances of use with respect to the operability or service life of the component. Two examples of this situation are cited below.
The connecting rod bearings used in modern engine construction consist of composite multilayer systems such as the structure comprising steel, bearing metal and optionally a third layer applied by electroplating or other surface coating methods. In virtually all cases, the starting material is coated strip material, to which the subsequent functional layer is applied by means of cladding by rolling, cladding by casting or sintering. After coating, the semi-finished product undergoes a plurality of rolling and/or annealing processes until, through metal-working processes such as pressing or rolling, the bearing shell assumes its final shape.
For this production method it is most common to use unalloyed, low-carbon, low-strength steels. The reasons for this are, on the one hand, that bearing pressing processes are technically difficult to control when substrates are used which comprise high-strength steels and, on the other hand, that in the case of low price products of this type the role of cost considerations, i.e. doing without the more expensive steels, is an ever more one. It is not possible, either, to fall back on work-hardened or surface-hardened backing materials in the production of such components, since it is impossible to maintain their relatively high initial hardness right through the individual manufacturing steps (e.g. annealing of the semi-finished product) to the finished bearing.
However, owing to the ever increasing specific loading of such components through minimisation of component size (weight reduction and thus lower fuel consumption) and/or the construction of ever more powerful engines, not only is it the case that very high demands are made of the bearing metal layer itself but also the steel backing is exposed to ever greater stresses. For example, in the case of steels exhibiting low wear resistance, such as are used today, any slight relative movements which may occur between bearing backing and connecting rod as-a result of high loading or speeds will lead to wear phenomena in the form of ridge formation or raised zones on the bearing backing. The consequences are a reduction in the ratio of bearing contact area to total area and thus in heat dissipation and the encouragement of oil carbon formation. These effects may lead, either individually or in combination, to defective functioning or even to complete failure of the bearing.
Another example would be so-called plate cams or disk cams, as are used in radial or axial piston machines for hydraulic pumps and motors. Conventional plate cams consist of composite materials, i.e. a steel backing with a functional layer of typical bearing metals such as for example CuPbSn alloys generally applied by means of cladding by casting. If, under heavy loads, relative movements occur in such components between the steel backing of the disk cam constructed as a composite multilayer material and the opposing machine component, cold welding may arise between the plate cam and its counterpart, which leads to impairment of the operation of the tribo-system or even to complete failure thereof.
DE-PS 688246 describes a method of hardening bearing shells made of iron alloys, wherein the overlay of the bearing shell is treated with a gas torch and the mould holding the bearing shell is simultaneously cooled. This method only affects the overlay.
DE-OS 2209148 relates to a method of energy radiation remelt treatment, with which refinement of material surfaces is carried out. The surface of narrowly defined, mutually unconnected surface elements is melted and then cooled in order to change the structure. In each individual instance, the choice of appropriate moulds and arrangements of the surface elements to be remelted of a workpiece or component depends on the respective applications and the stresses, wear problems and the like arising therefrom. However, no details are given as to the treatment of composite multilayer materials or, more particularly, of composite multilayer materials made up of low-melting-point metals, as used in sliding elements.
EP 01 30 175 A2 describes a plain bearing comprising a backing shell and a bearing metal layer applied to the steel shell, said bearing metal layer forming overlay zones distributed over the width of the overlay and extending at least substantially in the running direction, between which zones there are provided zones of a different hardness from that of the bearing metal layer. In order reliably to prevent the problems which may arise between the bearing materials of different hardnesses, heat treatment is carried out. Alloy materials are thus melted by means of laser and electron beams in the area of the zones extending in the running direction.
EP 01 30 176 relates to the use of laser and electron beams to improve plain bearings, but once again only the bearing metal layer is treated.